The architecture of a known computer I/O subsystem is illustrated in FIG. 1. An I/O bridge 1 was used to control data between the system bus 3 and the I/O bus 5. Multiple I/O peripheral devices were attachable to the I/O bus 5 such as a graphics device 7, SCSI (interface) 9, and/or a local area network (LAN) 11. Data transfer control between the peripheral devices connected to the I/O bus 5 and processing units connected to a system bus 3 was dominated by the I/O bridge 1.
An I/O bridge 1 included an inbound command queue 13 and an outbound command queue 15 which buffered the incoming commands issued by processors(s) such as a first central processing unit (CPU) 17, a second CPU 19, a third CPU 21 and a fourth CPU 23 of FIG. 1, each connected to system bus 3, and I/O device(s) connected to I/O bus 5. An inbound data buffer 25 and an outbound data buffer 27 contained in the data buffer unit were used for temporary storage of data transported between the system bus 3 and the I/O bus 5. Several control signal groups (system interface control signal line 29 and IOB interface control signal line 31) were issued by I/O bridge 1 to control the data buffer unit 4. Further, subsequent to issuance of the control signal groups, data channel 33 was used as a medium for data transfer.
FIG. 1 further illustrates an I/O control data bus 35 connecting I/O bridge 1 with I/O bus 5; a control bus 37 connecting I/O bridge 1 with system bus 3; and a data bus 39 connecting data buffer unit 4 with system bus 3. These buses 35, 37 and 39 are shown for completeness of FIG. 1.
I/O interface control signals 34 included "SYSCMD [x=0]", "IOCMD [y=0]" and "DATA [z=0]". SYSCMD [x=0] was used to control data transfer between the system bus 3 and the data transfer unit 4. IOCMD [y=0] was used to control the data transfer between the data buffer unit 4 and the I/O bridge 1. Finally, DATA [z=0] was used as a data medium for data transfer. Operation of the known computer I/O subsystem was as follows.
Each data transfer transaction on system bus 3 contained a data phase. The data transfer control signal group issued by the I/O bridge 1 contained signals driven in a data phase. These signals shown in Table 1 below, included the "data ready", "data bus busy" and "data" signals.
TABLE 1 ______________________________________ Signal Type Names Number ______________________________________ Data Ready DRDY 1 Data Bus Busy DBSY 1 Data D [x:0] X ______________________________________